1. Field of the Invention
The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a frequency converted light source for use with such inspection systems.
2. Description of the Related Art
The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Successive generations of such inspection systems tend to achieve higher resolution by illuminating the wafer or reticle using light energy having shorter wavelengths.
Certain practical advantages may be achieved when illuminating the wafer or reticle with light with wavelengths at or below 400 nm. Providing suitable lasers for high quality wafer and photomask inspection systems is particularly challenging. Conventional lasers generating light energy in the deep ultraviolet (DUV) range are typically large, expensive devices with relatively short lifetimes and low average power. Semiconductor wafer and photomask inspection systems generally require a laser generally having a high average power, low peak power, and relatively short wavelength in order to provide for inspection having sufficient throughput and adequate defect signal-to-noise ratio (SNR).
The primary method to provide adequate DUV power entails generating shorter wavelength light from longer wavelength light. This process of changing wavelengths is commonly called frequency conversion. Frequency conversion in this context requires high peak power light energy production in order to produce a nonlinear response in an optical crystal. To increase the efficiency of this process the longer wavelength light may have high average powers, short optical pulses, and may be focused into the optical crystal. The original light is typically called fundamental light.
High efficiency is important for a DUV laser. High efficiency allows a lower power fundamental laser source that is more reliable, smaller, and produces less heat. A low power fundamental laser will produce less spectral broadening if a fiber laser is used. Higher efficiency also tends to lead to lower cost and better stability. For these reasons, efficient frequency conversion to the DUV is relatively important.
Generating light at wavelengths below 400 nm, and especially below 300 nm can be very challenging. Light sources used for semiconductor inspection require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent. The lifetime, power, and stability of current DUV frequency converted lasers is generally limited by the frequency conversion crystals and conversion schemes, especially those exposed to DUV wavelengths like 355, 266, 213, and 193 nm.
Relatively few nonlinear crystals are capable of efficiently frequency converting light to UV/DUV wavelengths. Most crystals that have traditionally been employed have low damage thresholds if not properly prepared and the operating environment maintained. Thus the crystal has typically been contained within an enclosure to maintain the environment. In order to frequency convert an infrared laser to the DUV, more than one crystal can be employed. When multiple crystals are employed, it can be an advantage to place them all within the enclosure. Crystal alignment complications can result, and it can be difficult to collect and focus light in such an enclosure.
It would therefore be desirable to offer an enclosure that maintains the environment of the optical crystal and allows efficient frequency conversion at wavelengths at or below 400 nm. This efficient conversion may include multiple crystals of the same or different materials. Multiple frequency conversion steps may also be employed within a single enclosure. It is also important that any enclosure use materials that can provide increased lifetimes, stability, and/or damage thresholds as compared with designs previously available. In addition, it is desirable for an enclosure to allow pre-exposure processing of the crystal such as baking at high temperatures and allowing real time measurement of crystal properties.